As is known, sewing machines of the industrial type are provided with various auxiliary devices, the function of which is to make the sewing operations as complete and precise as possible, thereby also reducing the need for manual intervention by the operators controlling the sewing operations. Of these auxiliary devices, a particularly important role is played by the thread-cutting devices, the specific function of which is to cut off the thread chain, or loose portion of thread, which is formed on a fabric, at the ends of a seam, when the sewing machine continues to operate in the absence of fabric.
These devices suck in the free thread-chains by means of jets of compressed air. These jets can either strike the thread chains directly, so as to turn them back on the fabric and cause them to mesh with the seam, or may be directed, as is usual, so as to create a suction effect which draws the thread chains into a suitable suction mouth in the region of which a cutting element is arranged. The present-day thread-cutting devices perform the operations for which they are intended in an only partially satisfactory manner. In fact, calibrating the size of the suction mouth through which the thread chains are sucked still represents a delicate operation which is often inaccurately performed.
This suction mouth must in fact be relatively wide and located in the immediate vicinity of the edge of the fabrics being sewn, so as to ensure that the thread chains are sucked in. In this situation, however, it can easily happen that the fabric being sewn can accidentally enter the suction mouth and suffer extensive damage as a result.
Therefore, the operators controlling the sewing operations must carefully monitor the actual sewing and arrange the thread-cutting device so as to prevent, as far as possible, the fabrics being sucked in.
However, as is obvious, this gives rise, on the one hand, to the need to use an expert labor force and, on the other hand, to the acceptance of a large number of inaccuracies, during the sewing process, on account of incorrect calibration of the thread-cutting devices.
It is important to underline the fact that not all fabrics behave in the same manner with regard to the thread-cutting devices: the stiffer and heavier fabrics are substantially stable and are unlikely to enter the suction mouth of the thread-cutting devices, whereas the lighter fabrics are more likely to enter the latter.
The thread chains are also likely to enter into the suction mouth to a greater or lesser extent, depending on the thickness of the thread with which they are made.
Since this thread usually has a cross-section which is proportional to the thickness of the fabric being sewn, it follows that the thread chains present on stiffer fabrics require particularly large suction mouths, to ensure that they are sucked in, whereas the thread chains extending from lighter fabrics are sucked in even by smaller-sized mouths. It is obvious that a thread-cutting device intended for heavy fabrics will suck inside it not only the thread chain of lightweight fabrics, but also the lightweight fabrics themselves. On the other hand, a thread-cutting device designed for lightweight fabrics will prove to be ineffective for heavy fabrics. In view of this situation, the general object of the present invention is to design a thread-cutting device for a sewing machine, which is able to overcome the abovementioned drawbacks of the prior art.